"We are aware of who this person is, and didn't find any other chatter from him. As quick as he posted it, he pulled it off," said Lt. Michael Hoguet of Fairview Heights Police Department.

That Twitter handle was removed from the site late Tuesday afternoon.

Police said there has been no unrest at the mall in the wake of a fatal shooting in Ferguson, Mo., that has sparked unrest there. The tweet did not specify noon or midnight.

The social media rallying cry was quickly noticed by area law enforcement, who responded by verifying contingency plans for civil unrest. Fairview Heights police report normal staffing for patrols and administration.

"There have been no protesting, no looting, and no indications of any problems," Nick Gailius, Fairview Heights police chief, wrote on the department's Facebook page Tuesday afternoon.

At least two Fairview Heights patrol cars, including a canine unit, were parked at the mall Tuesday morning and early afternoon on the northwest and southwest areas of the parking lot, near Macy's and Sears' upper entrance. Officers said they were doing paperwork and using computers in the vehicles. An unmarked vehicle drove the perimeter of the mall.

Later, Illinois State Police cars also parked at or drove around the mall parking lot. There is a substation inside the mall, Hoguet said, but he said officers find their patrol cars more comfortable to complete reports.

Metro says it has increased security in the wake of unrest in North St. Louis County, but has not seen any increase in the number of calls to the Public Safety Department.

Richard Zott, public safety chief, said through the communications office that the extra security includes uniformed and not uniformed officers. He declined to provide further details.

Metro asks its customers to report any suspicious activity to any guard or call 314-289-6873.

Hoguet said officers have been monitoring social media and noticed the threat and retweets on Sunday about possible violence at the mall.

"We put (neighboring police departments) on notice about this low level social chatter," Gailius wrote on the Facebook page.

On Monday, the department reviewed contingency plans and alerted O'Fallon, Belleville and Illinois State Police to the possible threats. Those reviews, done periodically anyway, include contacting the fire department and public works departments to confirm communication protocol, Hoguet said.

"When we called yesterday everyone was ready, and ready to assist," he said.

The mall remains a "very safe place to shop, eat and visit," Hoguet said.